The present invention relates to a method for production of a branched polycarbonate composition, having increased melt strength, by late addition of branch-inducing catalysts to the polycarbonate oligomer in a melt polycondensation process, the resulting branched polycarbonate composition, and various applications of the branched polycarbonate composition.
Polycarbonate has excellent mechanical properties, such as impact resistance and heat resistance, and also has excellent transparency. It is widely used in many engineering applications. In one typical method for producing a polycarbonate, an aromatic dihydroxy compound such as Bisphenol A (BPA) and a diarylcarbonate such as diphenylcarbonate (DPC) are reacted in an ester exchange method in a molten state. This is referred to as the melt-polycondensation process or transesterification process. The resulting polycarbonate may be extruded or otherwise processed, and may be combined with additives such as dyes and UV stabilizers.
In certain processes and applications, such as blow moulding of bottles and extrusion of sheet products, it is desirable to use a branched polycarbonate due to the higher melt strengths of the branched polycarbonates. One method for making branched polycarbonates with high melt strengths is a variation of the melt-polycondensation process where the diphenyl carbonate and Bisphenol A are polymerized together with polyfunctional alcohols or phenols as branching agents. DE 19727709 discloses a process to make branched polycarbonate in the melt-polymerization process using aliphatic alcohols. Because this process requires expensive additional raw materials, such as the polyfunctional alcohols or phenols, additional costs and complexities are added to the polycarbonate production process. Additionally, it should be noted that continuous reactor systems require long periods of time and generate substantial amounts of transition materials when changing from one polycarbonate composition to another.
It is known that alkali metal compounds and alkaline earth compounds, when used as catalysts added to the monomer stage of the melt process, will not only generate the desired polycarbonate compound, but also other products after a rearrangement reaction known as the xe2x80x9cFriesxe2x80x9d rearrangement. This is discussed in U.S. Pat. No. 6,323,304. The presence of the Fries rearrangement products in a certain range can increase the melt strength of the polycarbonate resin to make it suitable for bottle and sheet applications. This method of making a polycarbonate resin with a high melt strength has the advantage of having lower raw material costs compared with the method of making a branched polycarbonate by adding xe2x80x9cbranching agents.xe2x80x9d In general, these catalysts are less expensive and much lower amounts are required compared to the branching agents.
JP 09059371 discloses a method for producing an aromatic polycarbonate in the presence of a polycondensation catalyst, without the use of a branching agent, which results in a polycarbonate possessing a branched structure in a specific proportion. In particular, JP 09059371 discloses the fusion polycondensation reaction of a specific type of aromatic dihydroxy compound and diester carbonate in the presence of an alkali metal compound and/or alkaline earth metal compound and/or a nitrogen-containing basic compound to produce a polycarbonate having an intrinsic viscosity of at least 0.2. The polycarbonate is then subject to further reaction in a special self-cleaning style horizontal-type biaxial reactor having a specified range of the ratio L/D of 2 to 30 (where L is the length of the horizontal rotating axle and D is the rotational diameter of the stirring fan unit). JP 09059371 teaches the addition of the catalysts directly to the aromatic dihydroxy compound and diester carbonate monomers. Typically, much higher levels of catalysts need to be added to the monomers to produce adequate branching and melt strength for blow moulding than for the production of general purpose linear polycarbonate. Also, if the catalysts or catalyst levels in the monomer mix tank of a continuous reactor system are changed, long operating times are required for the transition from the stable operation to produce linear polycarbonate to that to produce branched polycarbonate. This long transition time also produces large quantities of ill-defined transition materials having variable levels of branching. These disadvantages can be rather costly, especially when production plants frequently change between producing linear and branched polycarbonate.
There is a need for an improved process, which has rapid transition times and produces limited transition materials, for the production of a branched polycarbonate having increased melt strength.
The invention relates to a method for the production of a branched polycarbonate composition, having increased melt strength, by late addition of branch-inducing catalysts to the polycarbonate oligomer in a melt polycondensation process. Applicants have surprisingly found that by adding branch-inducing catalysts, such as alkali metal compounds and/or alkaline earth metal compounds, to the melt polycarbonate oligomer at a later stage of the melt polycondensation process, preferably after the oligomer has reached a weight average molecular weight (Mw) of between 3,000 and 30,000 g/mole, a unique branched polycarbonate composition is formed that has improved properties. It is believed that the addition of the branch-inducing catalysts at the later stages of the process produces a branched polycarbonate composition having longer chains between the branching points and containing less insoluble and unmeltable gels in the resin product, and thus a new composition is produced. The invention also relates to various applications of the branched polycarbonate composition.